1. Field of the Invention
The present invention relates to a process for the preparation of partially hydrogenated acrylonitrile-butadiene rubber (HNBR) in a pressurised reactor by hydrogenation of acrylonitrile-butadiene rubber (NBR) by means of homogeneous or heterogeneous catalysis with the application of Raman spectroscopy.
2. Description of the Prior Art
The partial hydrogenation of the C--C-- double bonds in acrylonitrile-butadiene rubbers (NBR) results in a special rubber, the hydrogenated nitrile rubber (HNBR).
In the hydrogenation processes carried out industrially at present, the NBR solutions are hydrogenated with hydrogen in an agitated pressurised autoclave in batches in the presence of a homogeneous or heterogeneous catalyst. The concentration of polymer in the solution to be hydrogenated is about 15 wt. %. In "Ullmann's Encyclopedia of Industrial Chemistry" dated 1993 [1], the homogeneous and heterogeneous catalysts used for the hydrogenation and the reaction conditions for the hydrogenation are described.
In the homogeneous hydrogenation both the catalyst and the substrate used for the hydrogenation are in solution. Chlorinated aromatic hydrocarbons such as, for example, chlorobenzene, are used as solvent. Rhodium-phosphine complexes or ruthenium-phosphine complexes are preferably used as catalysts. Depending upon the catalyst chosen and its concentration, the reaction temperatures are within the range of 100 to 150.degree. C. The reaction pressure, which is determined substantially by the hydrogen partial pressure, can vary from a few up to about 190 bar.
In the heterogeneous hydrogenation of NBR, palladium catalysts on, for example, carbon, calcium carbonate or silicon dioxide are preferably used, and the catalysts are dispersed in the dissolved substrate. The reaction is generally carried out in ketones as solvent at a temperature of about 50.degree. C. and at a pressure of about 50 bar.
Whereas sulfur or sulfur donors can be used for the vulcanisation of partially hydrogenated HNBR, the use of peroxide or high-energy beams is necessary for curing in the case of the completely hydrogenated product. Because of their good elongation at break and tear strength, commercially the partially hydrogenated HNBR types are preferred to the completely hydrogenated products.
A considerable problem, in particular in the production of the partially hydrogenated HNBR products, is the exact and reproducible establishment of the required degree of hydrogenation. It is known that the C--C-- double bonds of the 1,2-vinyl-configured butadiene units in NBR are hydrogenated very rapidly, followed by the 1,4-cis configured units. The 1,4-trans configured butadiene units are hydrogenated comparatively slowly. The NBR products used for the hydrogenation are distinguished by a predominant proportion of the 1,4-trans configured double bonds.
The progress of the hydrogenation can be found by determination of the hydrogen absorption or, more precisely, by infrared spectroscopic (IR) analysis of samples withdrawn from the reactor. An appropriate method of IR analysis is described in ASTM D 5670-95. The disadvantage of this procedure is that as a rule about 20 to 30 minutes elapse before the analytical results are available. During this period the reaction can already have continued over and beyond the desired end point. As the hydrogenation procedure requires expensive, pressure-resistant reactors, the economic efficiency of the entire process is also substantially dependent on the space-time yield. The economic efficiency of the hydrogenation process can be decidedly improved by increasing the product throughput while at the same time ensuring product quality according to specification.
NIR (near-infrared) spectroscopy is frequently used for the purpose of process control. As suitable optical fibres for NIR technology are available, the relevant NIR spectrometer can even be set up at a relatively great distance from the reactor. However, the disadvantage of NMR technology is that the fundamental vibrations of the IR spectrum are not measured, but rather the overtone and combination vibrations which are as a rule superimposed. Provided that the hydrogenation always proceeds under the identical conditions (temperature, polymer concentration, pressure), the degree of hydrogenation can be determined by means of chemometric methods. As technical processes always vary within a certain bandwidth, a reliable determination of the required reaction variables is not feasible.
The object was accordingly to find a new process which renders possible the establishment of a required degree of hydrogenation of HNBR with at the same time an improved spacetime yield.